[M-TT48] Frontiers in Geochemistry

[MTT48-P03] Development of ³H-³He groundwater dating method

*Nomata Naoki¹, Hirochika Sumino¹ (1.Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo)

Keywords:groundwater

Residence time of groundwater is an important parameter to elucidate groundwater flow system. We are developing the ³H-³He method, which is one of dating methods to estimate groundwater residence time ranging from several months to 120 years. In regions where water circulation is relatively active like those in Japan, this method is useful because it is considered that there are few groundwater with residence time more than several thousand years. The best advantage of this method is that it is applicable to accurately determine an age of young water compared to other methods (CFC & SF₆method and ³⁶Cl method, etc.), because initial ³H concentration is directly determined as the sum of ³He and ³H in the water at present, if the groundwater flow system has been a closed system for ³He and ³H until its discharge. The age determined by the ³H analysis only requires an assumption of the initial ³H concentration based on the record of the past precipitation, therefore it is difficult to determine unambiguous groundwater age (Mahara et al., 1993). In the ³H-³He dating method,³He concentration in a water sample is measured first by degassing of water in vacuum, and after a few months' storage, ³He produced from the decay of ³H in the sample is measured to determine³H concentration at the time of degassing. Thus, how the water sample is well degassed during the first analysis and how leaking of ambient helium into the degassed water is suppressed are crucial to determine small amount of ³H. We analyzed standard water samples provided by IAEA with known ³H concentrations to evaluate accuracy and precision of our method. When we compared the ⁴He concentrations of the samples analyzed just after degassing and those analyzed after storage for one month, the ⁴He concentration was extremely large in the samples stored for one month. As this indicates that the air helium intruded into the degassed sample during storage, we examined various ways to seal the sample container to prevent intrusion. This presentation will discuss developing of this method and how it will be applied to groundwater study in the future.